This is a tough one, fortunately help was available from Jenni Stockan of the Hutton Institue.

"I would say they are Nematus ferruginea, which is common enough in the north and west. There are two similar species - N. melanocephalus and N. cadderensis. The latter doesn't have the distinct black spots but more of a black line. N. melanocephalus has more of the black spots lying within the yellow patches and is a more southern species."

Nematus ferruginea is usually found on Betula, but there are reports of Salix being used.
So now we know and it is another new species for the Outer Hebrides list.